Fiber optic cables are widely used for telecommunications applications where high information capacity, noise immunity and other advantages of optical fibers may be exploited. Fiber cable architectures are emerging for connecting homes and/or business establishments, via optical fibers, to a central location, for example. One such architecture includes a trunk or main cable routed through a housing subdivision, for example, while small fiber count "drop cables" are spliced to the main cable at predetermined spaced apart locations.
A typical main cable may be installed underground and have multiple drop cables connected thereto, each of a hundred feet or more. Each of the drop cables, in turn, is routed to an optical network unit (0NU) serving several homes. Accordingly, information may be transmitted optically to the ONU, and into the home via conventional copper cable technology. Thus, the drop cables may serve groups of users, although other architectures may also employ a main cable and one or more drop cables connected thereto.
Unfortunately, the fibers within the main cable must typically be accessed at the various drop points and spliced to respective drop cables after the main cable has already been installed. Accessing the main cable for splicing requires careful preparation of the main cable including removing a portion of the cable sheath, and identifying and separating out predetermined fibers from within the cable without disturbing adjacent fibers. The separated fibers may then be spliced and secured within a conventional protective splice closure. Moreover, these cable access and splicing steps must typically be accomplished in the field by a technician who is likely to experience difficulties imposed by weather or the particular location of each of the drop points. Accordingly, field splicing of drop cables to a main cable is time consuming, expensive, and may produce low quality optical splices.
In an effort to overcome the disadvantages of field splicing drop cables at each of a series of drop points, so-called preterminated fiber optic cables have been proposed. A preterminated fiber optic cable includes a relatively high fiber count main cable to which respective low fiber count drop cables are spliced at predetermined drop points. The locations of the drop points are determined based upon field survey measurements.
The splicing of the drop cables to the main cable of a preterminated cable is performed at the factory during manufacturing of the cable. The preterminated cable, including the main cable, drop cables, and associated splice closures, are desirably wound onto a cable reel and delivered to the installation site. Accordingly, conditions for making high quality splices may be maximized in the factory, thereby increasing splice quality and also reducing the expense and difficulty associated with field splicing.
Exemplary of a preterminated cable is U.S. Pat. No. 5,121,458 to Nilsson et al. entitled Preterminated Fiber Optic Cable. The patent discloses a splice closure for splicing each of the drop cables to the main cable. The splice closure is generally cylindrical being no greater than 4 inches in diameter and 7 inches in length to facilitate winding of the preterminated cable, including the associated splice closures, onto a cable reel for shipping and installation. In particular, the patent discloses that the diameter of the splice closure is slightly less than 4 inches in diameter so that the preterminated cable may be installed through a 4 inch conduit.
The splice closure as disclosed in U.S. Pat. No. 5,121,458 uses a conventional technique for storing slack fibers within the splice closure, that is, the slack is stored in slack coils or loops. The slack coils must be made without violating the minimum bend radius of the fibers. Accordingly, the splice closure still must have a relatively large outer diameter which hampers winding of the splice closures onto the cable reel, and which may also greatly complicate installation of the cable within a section of buried conduit. For example, conduits less than 4 inches are commonly used for placing a conventional fiber optic or copper cable to facilitate installation under obstructions, such as a driveway. In addition, small diameter conduits are also desirably used to reduce excavation time and expense when installing the conduit.
U.S. Pat. No. 5,125,060 to Edmundson entitled Fiber Optic Cable Having Spliceless Fiber Branch and Method of Making Same discloses another approach in an attempt to overcome the difficulties in reducing the size of the connection or branching point of the drop cables from the main cable. The patent discloses spliceless stub cables extending from the main cable. More particularly, an opening is made in the jacket of the main cable at a disconnect point downstream from the intended branching point. The fibers to be branched for the stub cable are severed and pulled back through the branching point. The branched fibers are routed through a stub cable sheath and the disconnect point of the cable is sealed. The penetration into the main cable sheath, as well as its joint with the stub cable sheath are sealed at the branching point by a plastic housing. Thus, no splices are required and no slack optical fibers need be stored within the protective housing at the branching point.
Unfortunately, the approach disclosed in U.S. Pat. No. 5,125,060 suffers from a significant drawback in that the stub cable extending from the main cable is limited to only about 12 feet in length. Accordingly, for a typical fiber optic cable system route, a conventional splice is still needed to add an additional length of cable to the stub cable, and the splice must still be made in the field by a technician. Another drawback of the approach disclosed in U.S. Pat. No. 5,125,060 is that considerable care must be taken when pulling the 12 foot lengths of back through the branching point during preparation of the stub cables. A corresponding difficulty is encountered when stuffing the 12 foot lengths of fibers into the stub cable sheath. The main cable sheath is also penetrated 12 feet downstream from the branching point at the disconnect point, and this cable penetration must also be sealed to protect the cable from water entry.
Because the fiber optic cable systems described above include penetrations of the cable sheath, it is important to seal such penetrations to prevent water from entering the cable and damaging the optical fibers particularly at a splice location. One approach to sealing a splice point is disclosed in U.S. Pat. No. 5,125,060 which describes a heat recoverable housing surrounding the branch point from which the short stub cables extend. Similarly, U.S. Pat. No. 5,121,458 discloses a heat recoverable housing surrounding the splice closure which, in turn, stores the coils of slack cable in a conventional fashion.
U.S. Pat. No. 5,185,844 to Bensel, III et al. entitled Closure for Optical Fiber Connective Arrangements and Method for Making Same discloses a splice closure for joining together on/y two small fiber count cables in an in-line configuration with very little slack in the optical fibers. Thus, the closure is not suitable for splicing a drop cable to a main cable as in a preterminated fiber optic cable.